Seismic performance of rocking braced frames with double base plate connections

Abstract

This paper presents a rocking system for concentrically braced frames (CBFs) to mitigate the seismic demands induced by seismic motions. The proposed system incorporates double base plate connections (DBPCs) at the base of the braced-bay columns, enabling them to uplift and initiate a rocking motion. The DBPC comprised two base plates: the upper one is thin, connected to the column and designed to yield under uplift loading, while the lower one primarily distributes compressive loads to the foundation. The DBPC was modeled using a combination of spring and gap elements in SAP2000 software. A comparative study was performed to investigate the seismic performance of the proposed rocking system against a conventional CBF system. Key parameters such as the thickness of the upper base plate and the arrangement of anchor rods were considered. Four prototype frames of different heights (3, 6, 9, and 12 stories) were investigated under the Sylmar-x record using the nonlinear time history analysis. The results demonstrated that utilizing rocking braced frames with double base plate connections can remarkably mitigate the base shear force by softening the structural response, decreasing the seismic damage experienced by the structure. The upper base plate acted similarly to a footing damper, absorbing a substantial amount of energy by yielding. Importantly, the residual drifts caused by the plastic deformations of the upper base plate remained within the acceptable performance limits for self-centering systems. Moreover, the self-centering mechanism was achieved solely through the structure’s self-weight, eliminating the need for additional re-centering devices.